1. Field of the Invention
The present invention relates to an extreme ultra violet (EUV) light source apparatus to be used as a light source of exposure equipment.
2. Description of a Related Art
In recent years, as semiconductor processes become finer, photolithography has been making rapid progress to finer fabrication. In the next generation, microfabrication of 100 nm to 70 nm, and even microfabrication of 50 nm or less, will be required. Accordingly, in order to fulfill the requirement for microfabrication of 50 nm or less, for example, exposure equipment is expected to be developed by combining an EUV light source generating EUV light with a wavelength of about 13 nm and reduced projection reflective optics.
As the EUV light source, there are three kinds of light sources: an LPP (laser produced plasma) light source using plasma generated by applying a laser beam to a target (hereinafter, also referred to as “LPP type EUV light source apparatus”), a DPP (discharge produced plasma) light source using plasma generated by discharge, and an SR (synchrotron radiation) light source using orbital radiation. Among them, the LPP light source has the advantages that extremely high intensity close to black body radiation can be obtained because plasma density can be considerably made larger, that light emission of only the necessary waveband can be performed by selecting the target material, and that an extremely large collection solid angle of 2π steradian can be ensured because it is a point light source having substantially isotropic angle distribution and there is no structure surrounding the light source such as electrodes. Therefore, the LPP light source is thought to be predominant as a light source for EUV lithography requiring power of several tens of watts.
Here, a principle of generating EUV light in the LPP type EUV light source apparatus will be explained. By applying a laser beam to a target material supplied into a vacuum chamber, the target material is excited and turned into plasma state. Various wavelength components including EUV light are radiated from the plasma. Then, the EUV light is reflected and collected by using an EUV collector mirror that selectively reflects a desired wavelength component (e.g., a component having a wavelength of 13.5 nm), and outputted to an exposure unit. A multilayer film, in which thin films of molybdenum (Mo) and thin films of silicon (Si) are alternately stacked (Mo/Si multilayer film), for example, is formed on the reflecting surface of the EUV collector mirror.
In the LPP type EUV light source apparatus, the influence by neutral particles and ions emitted from the plasma is problematic especially when a solid target is used. Since the EUV collector mirror is located near the plasma, the neutral particles emitted from the plasma are deposited on the reflecting surface of the EUV collector mirror and reduce the reflectance of the mirror. On the other hand, the ions emitted from the plasma cut the multilayer film formed on the reflecting surface of the EUV collector mirror. The scattered materials from the plasma, including neutral particles and ions and the remains of the target materials, are called debris.
Recently, it has been confirmed that, with the combination of CO2 laser and solid tin target, the amount of debris generated from tin due to application of laser beam is significantly reduced. Further, as described in Murakami et al., “Conversion efficiency of extreme ultraviolet radiation in laser-produced plasmas” (American Institute of Physics, PHYSICS OF PLASMAS 13, 033107 (2006), pp. 7), regarding EUV light obtained by application of a laser beam generated by a CO2 laser to a metal target, it is becoming clear that high laser light/EUV light conversion efficiency (CE) can be obtained under the condition that the pulse width of the laser beam (FWHM: full width at half maximum) is on the order of 100 ns.
However, in a CO2 laser under study or a CO2 laser that has been commercially available, the pulse width of laser beam is about 1 ns to 50 ns. On the other hand, in a CO2 laser that performs pulse oscillation with RF modulation, the pulse width of laser beam is about several microseconds. A laser system that can realize the pulse width of about 100 ns with substantially homogeneous intensity has not been proposed yet.
As a related technology, Japanese Patent Application Publication JP-A-9-288251 discloses a pulse width elongation optics that reduces average energy within emission time by pseudo prolongation of emission time of pulse light. The pulse width elongation optics includes light dividing means for dividing pulse light along plural optical paths, and light synthesizing means for synthesizing the divided pulse light along the plural optical paths into pulse light along the same optical path, and the plural optical paths have predetermined differences in optical path from one another so that the intensity of the pulse light synthesized along the same optical path is substantially smaller than the intensity of the original pulse light before division along the plural optical paths. However, since the intensity gradually reduces as the pulse light is divided, it is difficult to generate pulse light with homogeneous intensity.